1. Field of the Invention
This invention relates to a sheet post-processing apparatus for aligning and stapling sheets discharged from an image forming apparatus, such as a copier, a printer or any other kind of recording apparatus, and to a stapling unit for stapling a bundle of sheets.
2. Description of the Related Art
A conventional sheet stapling device (hereinafter termed a stapling unit) includes a detachable staple cartridge providing a large capacity of staples. If staples within the staple cartridge are used up, the staple cartridge is replaced with a new one in order to supply staples.
The same configuration also is adopted in a sheet postprocessor (for example, a sorter, a finisher or the like) wherein postprocessing of sheets is automated by integrating a typical desktop stapler with an image forming apparatus.
Although the above-described conventional device has the advantage that suppling staples for the stapling unit is simplified by using the staple cartridge which can be easily replaced, the device has the disadvantage that since the staple cartridge is easily detached by anybody at any time, the staple cartridge might be carelessly detached.
That is, in general, in the stapling unit used in the sheet postprocessor, the staple cartridge is detached from the stapling unit only when a staple sensor provided within the stapling unit detects the absence of staples. In such a case, no problem arises if the staple cartridge is detached and a new staple cartridge is loaded. However, when the staple cartridge is detached from the stapling unit before it is empty, staples remain within the stapling unit as well as within the staple cartridge. When a new staple cartridge is subsequently loaded in the stapling unit, end portions of staples remaining within the stapling unit might disturb or turn up staples in the front end of the newly loaded staple cartridge. If the staple cartridge is forcibly set in the stapling unit in that condition, the disturbed staple belt might cause misfeeding of staples, jamming of staples, or the like.